Geochemical observations within the water column at the CO_2- rich hydrothermal systems Hatoma Knoll and Yonaguni Knoll IV, in the southern Okinawa Trough

摘要

The Okinawa Trough is one of three known hydrothermal sites worldwide where liquid carbon dioxide is emitted from the seafloor into the water column. In March 2008, investigations were performed at two active areas, Yonaguni Knoll IV and Hatoma Knoll, in order to identify impacts of hydrothermal venting on the water column chemistry. Vertical profiles of pH and redox potential (Eh) were recorded and discrete water samples were taken for the analysis of total carbon dioxide (C_T) and helium (~3He, ~4He). Anomalies with respect to reference stations (?C_T, ?pH) and ~3He with respect to saturation with the atmosphere (~3He~(excess)) were used to characterize the impact of hydrothermal vents. These data indicate that the flux of CO_2 into the water column is dominated by hot hydrothermal CO_2-rich vents located in close proximity to the liquid CO_2 emission sites. Bubbles and droplets sampled at the cold gas outlets at Hatoma Knoll differed considerably from the water column regarding CO_2/~3He ratios, and thus, provide additional evidence that cold liquid phase CO_2 is of minor importance for the total CO_2 flux at both hydrothermal systems. Although hydrothermal vents at back-arc basins are known to emit large amounts of acids other than CO_2, the correlation between ?pH and ?C_T at both research areas clearly suggests that the observed pH reduction is mainly caused by the addition of CO_2. Deviating ?CT/~3He and ?CT/?pH ratios and the prevailing water currents indicate a yet undiscovered vent site at the flank of a seamount in the northeast.